The invention concerns a process for inhibiting or retarding hydrate formation, growth and/or agglomeration in natural gas, petroleum gas or other gases, using at least one additive. Gases which form hydrates can comprise at least one hydrocarbon selected from methane, ethane, ethylene, propane, propene, n-butane and isobutane, and possibly H.sub.2 S and/or CO.sub.2.
Such hydrates are formed when water comes into the presence of a gas either in its free state or dissolved in a liquid phase such as a liquid hydrocarbon, and when the temperature of the mixture, including water, gas and possibly liquid hydrocarbons such as oil, drops below the thermodynamic temperature for hydrate formation, this temperature being fixed for a known gas composition and fixed pressure.
Hydrate formation is a problem, particularly in the gas and oil industry where hydrate formation conditions can be satisfied. One way of reducing the production costs of crude oil and gas both from the point of view of investment and exploitation, particularly in the case of offshore production, is to reduce or cut out treatments applied to the crude or gas to be transported from the field to the coast and leave all or part of the water in the fluid to be transported. Such offshore treatments are generally carried out on a platform located on the surface close to the field, so that the effluent, which is initially hot, can be treated before the thermodynamic hydrate formation conditions are reached due to cooling of the effluent with sea water.
However, in practice, when the thermodynamic conditions required for hydrate formation are satisfied, hydrate agglomeration causes the transport lines to block by creation of plugs which prevent the passage of crude or gas.
The formation of hydrate plugs can stop production and result in large financial losses. Further, restarting the installation, especially in the case of offshore production or sea transportation, can be a long process as the hydrates formed are very difficult to decompose. When the production from a submarine natural gas or oil and gas field containing water reaches the surface of the sea bed and is transported along the sea bottom, the reduction in the temperature of the effluent produced can mean that the thermodynamic hydrate formation conditions are satisfied and the hydrates formed bind together or agglomerate and block the transfer lines. The temperature on the sea bed can, for example, be 3.degree. C. or 4.degree. C.
Favourable conditions for hydrate formation can also be satisfied onshore when, for example, the ambient air temperature is low and the lines are not buried, or are not deeply buried in the ground.
In order to overcome these disadvantages, the prior art has sought to use substances which, when added to the fluid, can act as inhibitors by reducing the thermodynamic hydrate formation temperature. Such substances include alcohols such as methanol, or glycols such as mono-, di- or tri-ethyleneglycol. Such a solution is very expensive as the quantity of inhibitors to be added can be as high as 10% to 40% of the water content and those inhibitors are difficult to recover completely.
Insulation of the transport lines has also been recommended, to prevent the temperature of the transported fluid from reaching the hydrate formation temperature under the operating conditions. This type of technique is also very expensive.
The use of additives which can modify the hydrate formation mechanism has also been recommended, in which instead of agglomerating rapidly with each other and forming plugs, the hydrates formed disperse in the fluid without agglomerating and without obstructing the lines. Examples are: our European patent application EP-A-0 323 774, which describes the use of non-ionic amphiphilic compounds selected from the esters of polyols and carboxylic acids, which may or may not be substituted, and compounds with an imide function; our European patent application EP-A-0 323 775, which describes the use of compounds from diethanolamides of fatty acids or fatty acid derivatives; U.S. Pat. No. 4,856,593 which describes the use of surfactants such as organic phosphates, phosphate esters, phosphonic acids, and salts and esters thereof, inorganic polyphosphates and esters thereof, and homopolyacrylamides and acrylamide-acrylate copolymers; and European patent application EP-A-0 457 375, which describes the use of anionic surfactants such as alkylarylsulphonic acids and their alkali metal salts.
Amphiphilic compounds obtained by reacting at least one succinic derivative selected from the group formed by polyalkenylsuccinic acids and anhydrides with at least one polyethyleneglycol monoether have also been proposed for reducing the tendency of natural gas hydrates, petroleum gas hydrates or other gas hydrates to agglomerate (European patent application EP-A-0 582 507).
The use of additives which can inhibit or retard hydrate formation and/or growth has also been recommended. Examples are European patent application EP-A-0 536 950 which describes the use of tyrosine derivatives, International patent application WO-A-93 25798 which describes the use of homopolymers and copolymers of N-vinyl-2-pyrrolidone and mixtures thereof, International patent application WO-A-94 12761 and U.S. Pat. No. 5 432292 which describe the use of poly(N-vinyl-2-pyrrolidone), hydroxyethyl-cellulose and mixtures thereof or a terpolymer based on N-vinyl-2-pyrrolidone, N-vinyl-s-caprolactame and dimethylaminoethyl methacrylate sold under the trade name GAFFIX VC-713. International patent application WO-A-95 19408 more generally describes the use of aliphatic polymers containing carbonylated N-heterocycles in complex formulations. This is also the case in International patent application WO-A-95 32356, which describes the use of terpolymers based on N-vinyl-2-pyrrolidone, acrylamidomethylpropanesulphonate and acrylamide. Finally, International patent applications WO-A-95 17579 and WO-A-96 04462 describe the use of allylated ammonium, sulphonium and phosphonium derivatives, used either alone or mixed with a corrosion inhibitor.